Tombstone fixtures are widely used for supporting one or more workpieces clamped against faces of the fixture, while the workpieces are machined. This basic fixture generally comprises a tooling column rising from a fixture base, and the base is adapted to fit a machine support surface, such as a machining center table. In many production environments, the fixtures are affixed to the top surface of interchangeable machine pallets which have common registration and clamping surfaces mated to master machine registration and clamping surfaces. Also, the fixtures are often provided with permanent locating and clamping hardware, and are said to be "dedicated" to the production of a specific part. Several examples of prior art tombstone fixture configurations are depicted in FIGS. 1a-d, herein. FIGS. 1a and 1b are four-sided fixtures, with FIG. 1a being solid, and FIG. 1b being hollow. FIGS. 1c and 1d are T-type fixtures, each resembling an inverted T, with FIG. 1c being solid, and FIG. 1d being "windowed" by a through aperture. An example of a tombstone fixture fully rigged with clamping and locating hardware for supporting a plurality of workpieces during machining operations is shown in U.S. Pat. No. 5,454,148, of G. A. Ewer, et al, issued Oct. 3, 1995, entitled Tombstone Fixture. Further examples of tombstone fixtures are shown in U.S. Pat. No. 3,790,153, of H. Seidenfaden, issued Feb. 5, 1974, entitled Equipment for Holding Workpieces, and U.S. Pat. No. 4,630,811, of C. D. Rudisill, issued Dec. 23, 1986, entitled Modular Fixturing Apparatus. Examples of interchangeable machine pallets are shown in U.S. Pat. No. 4,468,019, of S. Staudenmaier, issued Aug. 28, 1984, entitled Pallet Clamping System for Machine Tools, and U.S. Pat. No. 4,512,068, of T. W. Piotrowski, issued Apr. 23, 1985, entitled Pallet Receiver with Compliant Pin and Socket Registration.
In machines for performing operations on a workpiece, especially those operations where stock is removed from the workpiece, vibrations are frequently generated. One such machine for removing stock is a CNC machining center, which is a machine for milling, drilling, etc., in an automatic cycle, and with automatically interchangeable tools (e.g., see U.S. Pat. No. 4,512,068, above). Tuned damped absorbers, often referred simply as "dampers", comprise a body of art for introducing structural damping where desired. But dampers are typically applied in structural locations not subject to changes which would effect frequency response. Thus, dampers have not, traditionally, been applied to the workpiece fixture side of a machining center, for example, because of the potential changes in mass of the different workpieces which are machined.
With regard to machine tool subassemblies, a prior art damper has been applied to the wheelhead of a grinding machine; such damper is found in U.S. Pat. No. 3, 522,864, of R. C. Richter, issued Aug. 4, 1970, entitled Tuned Vibration Damper Assembly for Machine Tools. In this Patent, a wheelhead is rockable about a pivot point, and a tuned damper is mounted to an angled face of the wheelhead. The damper comprises a damping mass, a resilient member between the wheelhead and damper mass, and a bolt and spring combination for securing the mass to the wheelhead face in a manner preloading the resilient member. The resilient member acts in shear along the angled face, providing components of shear damping in the X and Y direction, which counteract moments of vibration about the pivot point.
With regard to the "cutter side" of a machine, i.e., as opposed to the workpiece fixture side of the machine, a rotary damper has been applied to the cutter arbor of a horizontal milling machine. This damper is depicted in U.S. Pat. No. 2,714, 823, of A. H. Dahl et al., issued Aug. 9, 1955, entitled Vibration Damper.
Damping has also been applied to a boring bar support of a turning machine, as shown in U.S. Pat. No. 5,033,340, of L. T. Siefring, issued Jul. 23, 1991, entitled Apparatus and Method for Tool Vibration Damping.
It is also known to place a damper inside a boring bar which supports a cutter. U.S. Pat. No. 3,447,402, of J. C. Ray, issued Oct. 25, 1967, entitled Damped Tuned Boring Bar, shows such a structure.
With regard to the "workpiece side" of a machine tool, an example of a vibration damping apparatus applied directly against a stationary workpiece is shown in U.S. Pat. No. 5,494,269, of P. E. McCalmont, issued Feb. 27, 1996, entitled Vibration Damper. In this structure, a damping unit is mounted to a machine table and is manually actuated to load a damping head against an otherwise unsupported cantilevered workpiece portion; the damping head is retracted when the workpiece is to be removed from the machine.
Another a workpiece damper is depicted in U.S. Pat. No. 4,438,599, of J. T. Kamman et al., issued Mar. 27, 1984, entitled Vibration Damper for Machine-Carried Workpiece. In this patent, a shoe held in a shoe holder by viscoelastic pads is applied directly against a slender rotating workpiece, to attenuate workpiece vibrations.
The present invention is applicable to the workpiece side of a machine tool, and is particularly pertinent to those situations where the workpiece mass comprises a small percentage of total mass of fixture and workpiece, and where the change in total mass as the workpiece is machined, is also small. The invention obviates many of the difficulties inherent in the prior art devices, by incorporating a tuned damper into a tombstone fixture for supporting workpieces.